This invention relates to snap action electrical switches. It is particularly concerned with a snap action switch capable of carrying currents loads on the order of 20 amps or so. Switches having this current capacity require a minimum contact force of about 100 grams to ensure a low resistance contact interface. They also require a minimum air gap between contacts of about 1 millimeter to prevent arcing. Given these parameters, making a reliable snap action switch having high current capacity, low resistance, small size and long life becomes a difficult undertaking as satisfaction of one criterion conflicts with meeting the next.
One aspect of prior art snap action switches is the use of a spring blade having two stable or rest positions. These blades are made of flat, resilient material. They have an elongated central compression member and two elongated tension members on either side of the compression member. The three elongated members have their ends joined at unitary contact pads. The tensions members impart a curved or bowed configuration to the compression member. When a user actuates the switch a plunger or the like acts on the blade, eventually moving the compression member through the center of its bowed shape, whereupon the blade snaps to a thrown position. Having been moved over center, the blade assumes a second stable or rest position. Thereafter, returning the switch to its initial condition requires application of force to move the spring blade back through center to its first stable position. Provision of a mechanism to effect this returning force complicates the switch design, thereby raising its cost.
The complications of a return mechanism can be avoided by not allowing the spring blade to snap all the way over center. In other words, if the size and spacing of the switch parts are carefully controlled, the spring blade can be arranged to snap to a thrown position without moving through center. Then when the actuating force is removed the blade returns on its own to its initial condition. This approach is used in prior art switches such as Rose, U.S. Pat. No. 4,523,064.
The spring blades in these prior switches still have two stable positions but the switch housings are designed to constrain the blade from ever moving far enough to reach the second stable position. The problem with restricting the spring blade travel to prevent over-centering is a marked increase in the number of critical dimensions to assure proper functioning of the switch. That is, the shape and mechanical properties of the blade must be carefully controlled. The base in which the blade is mounted must support it precisely, the fulcrum has to be located exactly, the actuator must consistently impinge at the right place, and so on throughout the structure. Holding these tolerances becomes as much trouble as providing a return force for an over-centered blade. The switch of the present invention overcomes these problems with a spring blade that has only one stable position.